Chore: Enable warnings as errors on MSVC (#6456)

* tests: add Sanity test for SplitFilename83

fix test

fix test

* disable `C4715:not all control paths return a value` for nihstro includes

nihstro: no warn

* Chore: Enable warnings as errors on msvc + fix warnings

fixes

some more warnings

clang-format

* more fixes

* Externals: Add target_compile_options `/W0` nihstro-headers and ...

Revert "disable `C4715:not all control paths return a value` for nihstro includes"
This reverts commit 606d79b55d3044b744fb835025b8eb0f4ea5b757.

* src\citra\config.cpp: ReadSetting: simplify type casting

* settings.cpp: Get*Name: remove superflous logs

src/citra_qt/bootmanager.cpp

@@ -338,6 +338,7 @@ struct SoftwareRenderWidget : public RenderWidget {
                     case GPU::Regs::PixelFormat::RGBA4:
                         return Common::Color::DecodeRGBA4(pixel);
                     }
+                    UNREACHABLE();
                 }();
 
                 image.setPixel(x, y, qRgba(color.r(), color.g(), color.b(), color.a()));